In U.S. Pat. No. 4,807,031 issued to Broughton et al. on Feb. 21, 1989, subliminal encoding of digital information in a television signal is described. Like the present invention, in the patentee's arrangement, the encoded information can then be recovered in a receiver that is not connected to the television on which the television signal is displayed, but rather that "sees" the television image and captures the information. Once captured, the information is later be used to control the activities of a device that is part of and attached to the receiver, such as a toy.
The Broughton arrangement has several difficulties. As an example, subliminal encoding is accomplished by selectively increasing the intensity of one of each pair of adjacent scan lines in each frame of the video image. For example, if, in one frame, adjacent scan lines are of unequal intensity, this may signify an encoded "1", while if the adjacent scan lines have the same intensity, this may signify a "0". The problem with this approach is that the encoded information causes changes in the frequency domain representation of the video portion of the television signal, which occur at one-half of the line rate. Since there is a strong signal present at this frequency even without data present, this makes the encoded information difficult to capture in the receiver.